Group III nitride semiconductors, which cover a wide band gap by means of combinations of group III elements such as Ga, Al, and In, are widely used in optical semiconductor devices such as light emitting diodes (LEDs) and laser diodes (LDs) and electronic devices for high frequency and high-power applications. These group III nitride semiconductor-containing devices are generally produced by epitaxially growing a group III nitride semiconductor layer on a sapphire substrate. However, the sapphire substrate has a large lattice mismatch of 13.8% with group III nitride semiconductors (GaN). The lattice mismatch is represented by {(Lattice constant of GaN−Lattice constant of sapphire)/Lattice constant of GaN}. Thus, there is a problem of a high defect density when a group III nitride semiconductor is epitaxially grown on the substrate. As a novel substrate to reduce the lattice mismatch described above, a ScAlMgO4 substrate has been suggested, and a GaN epitaxial technique including the substrate has been disclosed Japanese Patent Application Laid-Open No. 2015-178448. ScAlMgO4 has a small lattice mismatch of −1.8% with GaN. The lattice mismatch is represented by {(Lattice constant of GaN−Lattice constant of ScAlMgO4)/Lattice constant of GaN}. Accordingly, a group III nitride semiconductor epitaxially grown on a ScAlMgO4 substrate is likely to provide high quality and high performance, and development of such semiconductors into various group III nitride semiconductor devices is anticipated.
The ScAlMgO4 substrate disclosed in Japanese Patent Application Laid-Open No. 2015-178448 is obtained by cleaving a ScAlMgO4 single crystal along a C-plane ((0001) plane). Meanwhile, it is known that epitaxially growing a group III nitride semiconductor along a step edge as a level difference of atomic layer level existing on the substrate surface (hereinafter, also referred to as “step flow growth”) provides a high-quality film. The surface of the ScAlMgO4 substrate cleaved along the (0001) plane aforementioned, however, has no ideal step. Thus, the group III nitride semiconductor itself causes random nucleation on the (0001) plane during epitaxial growth, and step flow growth is induced along such nuclei. Such growth from random nuclei, which exhibits random growth directions, has a problem in that the group III nitride semiconductor surface results in rough surface morphology having large waviness.
The ScAlMgO4 substrate having a (0001) plane produced by cleavage as the main plane includes areas having large irregularities of several hundred nm or more, depending on the cleavage accuracy, on the main plane. These large irregularities may cause faults such as defect formation in epitaxial growth.
Meanwhile, in group III nitride semiconductor growth using a sapphire substrate having a similar hetero structure, periodic steps are formed by allowing the substrate to have an off-angle to thereby step flow-grow a group III nitride semiconductor. There is disclosed use of a high-performance group III nitride semiconductor produced on such a substrate having an off-angle for formation of an LD (Japanese Patent Application Laid-Open No. 2008-98664).